UPSes and Generators
UPS is an abbreviation for Uninterruptible Power Supply. In short, a UPS creates AC power from an internal battery and an inverter, when the main source of AC power fluctuates outside of normal limits (or completely dies off). In most cases, during normal operation, the AC power source is passed straight through the UPS to the outlets. When the source of AC power fails, the UPS begins generating AC power and quickly switches the outlets to the internal inverter.
The inverters inside UPSes are categorized by the type and quality of the power they generate. There are two primary inverter designs inside UPSes: modified sine wave, and pure sine wave. Modified sine wave designs are easier to produce and are subsequently cheaper, but produce electricity that is less desirable. Pure sine wave UPSes produce electrical power that nearly matches the power provided by the utility company.
The Baseline: Utility Power
To start of this project, I wanted to get a good look at what the utility power looks like as it come into my office, to serve as a baseline of what to expect.
Although not bad, I couldn’t help but notice the distortion in the save at the top and bottom of each cycle (the tops and bottoms are flat, and there is a slight change in angle just before each peak and valley).
CyberPower 850VA UPS
The first UPS that I tested is the CyberPower 850VA. This is an offline UPS that uses a modified sine wave inverter. Because it’s an offline UPS (power is passed straight through while in normal range), it’s no surprise that the output from the UPS looks identical to the utility power:
When the source of electricity is removed, the UPS immediately kicks in and begins generating electrical power. However, notice how it does not match the utility company power. The inverter creates output that has particularly flat tops and bottoms, with pretty sharp points at the leading edge of the cycle. It’s really not a sine wave at all.
The nature of this type of switching is tolerated well by resistive loads like incandescent lamps and heating elements, but is especially harsh on electronic devices that use switching power supplies (most stuff now-a-day).
CyberPower 1325VA Pure Sine Wave UPS
The CyberPower 1325VA UPS is advertised as producing a pure sine wave from its inverter. I did the same tests on this unit as the previous. This UPS is also an offline UPS, so while utility power is available, it is passed straight through to the outputs, as shown here:
Unlike the other UPS, however, when utility power is removed, this UPS creates nearly identical power to what the utility provides. In fact, I think it’s arguably a better sine wave than what the utility is getting to me. (Notice the tops and bottoms of each cycle are much more even than utility power):
Since I had everything setup, I decided measure the output from my gas powered generator, a Honda EU2200i. This generator uses an inverter to create pure sine wave power output. I wanted to see for myself how close to utility power this generator can get.
This is the output from the generator, while idling, with nothing plugged in (i.e no load):
Sometimes the output changes slightly when the inverter get loaded down. I grabbed one of my 500 watt utility lights and plugged in, then scoped the output again. Still a nice pure sine wave.
Then I turned off the idle mode to force the engine to run at full throttle. I wanted to see if that changes the output of the inverter any. Nope, no change.
And finally, the same full throttle test with a 500 watt resistive load attached. Still a perfect sine wave.